There is a growing demand for sliding switches that use printed circuit boards, wire frames, and the like as stationary contacts. Such switches are used in vehicles (e.g., to control lights, turn signals, etc.), in household devices (e.g., as program switches for washers and dryers, etc.), and many other applications.
A conventional arrangement and structure of contacts of a sliding step switch is shown in FIGS. 12–14. The arrangement depicts a three function configuration 510 for a sliding switch. A circuit board substrate 512 is formed of a synthetic resin made of an insulating material. A first conductive stationary contact pad 514 connected to a positive terminal of a power source is disposed on substrate 512. Second, third, and fourth conductive stationary contact pads 516, 518, 520 connected to a negative terminal of a power source through an electrical load via a ground connection are disposed on substrate 512. An insulating material 522 such as a solder mask is disposed between contact pads 514, 516, 518, 520.
A movable contact assembly 524 is mounted to an unillustrated holder which permits movement in the directions indicated by arrows A and B. Movable contact 524 includes first and second cylindrically shaped movable conductive contact heads 526, 528, mounted to respective conductive contact springs 530, 532. Contact springs 530, 532 are connected together by a conductive metal strip 534.
As shown on FIG. 12, movable contact assembly 524 is in a first steady state position enabling current to flow from first contact pad 514 through movable contact 524 into second contact pad 516 to activate the function controlled by second contact pad 516. As movable contact assembly 524 moves along a path in parallel with the direction of arrow B movable contact heads 526, 528 moves to other positions where various functions are activated or deactivated. Likewise, movable contact assembly 524 can also move along a path in parallel with arrow A.
Electrical contact is made between a cylindrically shaped movable contact head and a flat stationary contact pad by pressing the contact head onto the stationary contact pad creating a line of electrical contact points. Upon operation of the switch, contact is broken by movement of the movable contact head past the edge of the stationary contact pad, a line of electrical contact points being maintained until just before breaking the contact.
Under specific voltage and current conditions, an arc is initiated at the last point of electrical contact as the electrical contacts are moved apart from each other. The current flowing through the gap between contacts generates heat, resulting in temperatures high enough to cause arc erosion; some of the nearby insulation may be burned away.
FIG. 13 illustrates an electrical schematic of the switch configuration shown on FIG. 12. FIG. 14 shows a sectional view of the switch configuration shown on FIG. 12.
FIG. 15 illustrates the area 546 on a conventional contact pad where arcing occurs. This area is known as an arcing zone. During the life of the switch, debris fields 548 including both conductive and insulating material build up on the stationary contact pads and insulating regions as a result of arc erosion.
Sliding movement of the contact head through the debris field also causes debris particles to be dragged into a main or steady state area of contact, known as a contacting zone 542, on the stationary contact pad 520 resulting in increased contact resistance when the contact head electrically contacts the contacting zone on the stationary contact pad during steady state use of the switch. The switch fails when debris causes the resistance between contacts to increase to a level whereby the contacts can no longer effectively complete a circuit or resistance becomes unacceptably high. FIG. 16 illustrates a graph showing voltage drop across contacts as a function of switching cycles of a conventional switch. In the illustrated example, voltage begins to increase and become unstable after about 25 arcing cycles.
During switch operation, debris particles are also dragged onto insulating material disposed between stationary contact pads as the contact head is moved from one contact pad to another. Debris on the insulation material reduces the dielectric strength of the insulation. The switch fails when the isolation resistance between the contact pads is reduced to a point where a circuit is established between contact pads. Lubrication of the contacts generally increases the rate at which debris is deposited onto the insulation.
As electrical performance requirements for sliding switches continue to increase, improvement in sliding switch performance is needed to satisfy increasingly stringent requirements.